Losses in Steam lines

[alka5eltzer]

Dear all,
I’m looking to find the approximate heat/steam losses in steam lines.
I want to justify the instalation of lagging/insulation.
• I know the diameters of the various lines and the lengths.
• I know my steam temperature (175 degC) and pressure (9 Bar).
• I know the average monthly wind speed and ambient temperatures.
How can I work this out… is there a formula?
Thanks loads.
Alka

 

[jmiles]

Sure, there is a simple way to approach this one, set up a thermal circuit, for simplicity i would just assume your steam to be a constant temperature fluid int he pipe, then you can either calculate or assume an internal convection coefficient. work out the conduction throught he pipe, and then the external convection coefficient. then repeat the process for the system with insulation.

q = T1 - T2 / (R)

Where T1 is the inside temperature,
T2 is the outside temperature
R = R1 + R2 + ... Rn
R1 = Internal convection resistance = 1/(2Pi*L*h)
Where L = Length of pipe
h = Internal convection coefficient
R2 = Conduction resistance through pipe or insulation or whatever = ln(r2/r1)/(2Pi*k*L)
Where r2 is the outside radius of the pipe,
r1 is the inside radius
k is the thermal conductivity
L is the length

Please note that Rn will be of the form of R1 except using outside convection coefficient.

 

[Latexman]

Large companies tend to have standards on this, i.e. it's already worked out.

Small companies probably look to their insulation supplier to provide guidance.

Most good college heat transfer textbooks (Holman or Kern) and engineering handbooks (Perry's or Mark's) have the formulas you need. Forced convection on the outside due to the wind. Condensation on the inside - I'd probably try to use the Dukler plots first. Conduction through the steel and insulation. Remember insulation comes in certain specific thicknesses, this is not a continuous function problem.

Good luck,
Latexman

 

[stgrme]

You can dowload a free program, "3E Plus", from the North American Insulation Manufacturers Association (NAIMA) at the following web site:

http://www.pipeinsulation.org/

You already have the input for the software. You can run cases with or without lagging. Options are available for different lagging material and surface conditions.

Best of luck!

 

[alka5eltzer]

Hi all,

Thanks loads for your help... brilliant. I love the calculator at:

http://www.pipeinsulation.org/

I've been looking at this page:

http://www.spiraxsarco.com/resources/steam-engineering-tutorials/steam-distribution/air-venting.asp

It seems pretty good... But i'd like to know how they got the values in table 10.5.1?
It assumes:
"Heat emission from bare horizontal pipes with ambient temperatures between 10°C and 20°C and still air conditions"...

I'm thinking the same but I'd like to change the ambient temperatures... I know that where I am my average ambient temp in Jan is 4DegC, Feb its 4.5DegC, March its 6DegC and so on.... (my aim is to plot a graph showing losses over the year).

Does anyine know how they got the values in the table?

Also... they mention Enthalpy of evaporation (hfg) = 2048 kJ/kg... how did they find this?

Thanks loads again